Effects of chemical additives and mature compost on reducing nitrogen loss during food waste composting

This study is aimed at adding different types of mature compost and sulfur powder, as additives into food waste composting to investigate the effect on nitrogen loss and compost maturity. The composting experiment used the in-vessel composting method and was conducted continuously for 15 days. High-throughput sequencing was used to analyze the bacterial community during composting. Results showed that the secondary fermentation mature compost mixed with sulfur powder group had the most reduction of ammonia emission (56%) and the primary fermentation mature compost amendments were the most effective for nitrous oxide emission reduction (37%). The temperature, pH, and nitrogen forms of transformation of the pile significantly affect the nitrogen loss during composting. Firmicutes helped to promote the rapid warming of the pile, and Actinobacteria and Proteobacteria played an important role in decomposition of organic matter. Thermobifida and Ureibacillus had a main contribution to the rapid degradation of organic matter in the process of composting. The relative abundance of nitrogen-fixing bacteria was higher, and the relative abundance of predominantly ammonifying and denitrifying bacteria was lower than the control group, with the addition of different additives.


Introduction
With the rapid development of society, the production of food waste (FW) in China is increasing, and the annual production of FW reached 60 million tons every year (Wang et al. 2022). FW have been regarded as valuable substances due to their enrichment of biodegradable organic substances (Karimi and Karimi 2018). Composting is a pragmatic process for organic waste treatment. During composting, microorganisms can transform organic wastes to nontoxic and nutrient-rich biofertilizers to improve soil fertility, thereby enhancing crop yield and quality Zhan et al. 2022). During composting, microorganisms can transform organic wastes to nontoxic and nutrient-rich biofertilizers to improve soil fertility, thereby enhancing crop yield and quality. However, FW composting is impeded by several technical challenges, such as gaseous emissions and immaturity, which not only reduce the availability of produced biofertilizer but also lead to severe secondary environmental pollution (Yang et al. 2013;Wei et al. 2017). FW is rich in nitrogen content (up to 4% of dry matter), which is highly susceptible to 26%-85% total nitrogen loss during composting processes, and ammonia (NH 3 ) emission accounted for more than half of the total nitrogen loss (Kim et al. 2018).
Additives have been widely used to improve composting performance and reduce nitrogen loss by regulating matrix Responsible Editor: Diane Purchase Shangao Xiong and Yongdi Liu contributed equally to this work. physiochemical properties and succession of bacterial community. Additives such as biochar and zeolite contain more pore structure and larger specific surface area, which can adsorb nitrogenous gases, improve the structure of the pile, and promote the composting efficiency (Wang et al. 2018b). The cornstalks and wood peat contain more pore structure and larger specific surface area, which can adsorb nitrogenous gases, decreased NH 3 emissions by 55.8% and 71.7%, respectively (Yuan et al. 2018). Yang et al. (2013) reported that adding bulking agents to compost feedstock significantly reduced nitrous oxide (N 2 O) emissions and reduced nitrogen losses. As a commonly used additive, mature compost has a soft texture and large specific surface area and contains a large number of microorganisms, which can effectively shorten the composting time and have a better deodorizing effect (Maeda et al. 2018). The mature compost as an amendment is more accessible with lower cost, while after secondary fermentation of composting, the thermophilic microorganisms in the material are retained, which is more conducive to the rapid warming, dewatering, and maturation of the pile. In addition, the mature compost has a strong moisture retention and adsorption capacity, which can delay the release of gas during composting (Abichou et al. 2009). However, the mechanism of mature compost for reducing nitrogen losses in composting process and the effect of different levels of mature compost on the degree of reduction of nitrogen loss were unclear.
Chemical additives regulate the nitrogen loss during composting mainly by adjusting the pH of the pile and by fixing ammonia nitrogen (NH 4 + -N) (Wu et al. 2019). The pH between 6.0 and 8.5 can promote microbial activity in composting, and the functional microorganisms of nitrogen are susceptible to the pH of the substrate. The change of pH in composting is important on the nitrogen conversion and loss (Cao et al. 2020). NH 4 + -N in the pile exists in the form of ammonium salts in the pile through chemical action, so many acidic additives (calcium superphosphate, phosphate, etc.) will play a significant role in nitrogen fixation when added to the pile. As an acidic chemical, sulfur powder (SP) can effectively reduce the pH of the compost pile and has a significant effect on NH 3 emission inhibition, which can effectively reduce nitrogen loss. Gu et al. (2018) set up experiments with different ratios of SP addition and found that the addition of SP could significantly reduce nitrogen loss during composting. However, the synergistic effect of SP and mature compost on reducing nitrogen loss during composting was less studied.
In this study, different types of mature compost amendments and SP were added into FW composting. The composting performance and the nitrogen component transformation were comprehensively evaluated. The evolution of physiochemical characteristics and emissions of NH3 and N 2 O during composting were measured. The response of bacterial community to diverse additives was explored by high-throughput sequencing and bioinformatics methods to reveal the microbial mechanism with organics degradation and nitrogen loss affected by different additives.

Preparation of raw materials
The FW, mature compost, and SP were obtained from organic recycling research institute (Suzhou) of China Agricultural University (Suzhou, China). Nondegradable substances, such as glasses, metals, and plastics, were manually removed from FW. The sawdust was purchased from a lumber mill in Suzhou (particle size < 2 mm). Sulfur powder is light yellow powder, and the properties were as follows: melting point is 112.8 °C, density is 2.36 g/cm 3 , purity is 99.999%, powder size is 200 mesh, and the sum of all conductive metal impurities is less than 3 ppm. Mature compost was obtained from China's first urban and rural organic waste treatment and utilization demonstration center in Linhu town, Suzhou city (Wei et al. 2021). The raw materials were dewatered by biodrying at various small treatment sites in Wuzhong District, Suzhou city, and then sent to the large-scale silo composting reactor for 10-day composting, and the germination index (GI) could reach more than 70%, reaching Chinese National Standard (NY/T 525-2021). The primary fermentation mature compost (1B) was the product of 10-day composting. The secondary fermentation mature compost (2B) was the product of 20 days of static stacking composting of 1B. Physiochemical properties of these raw materials based on the dry matter (DM) are summarized in Table S1.

Experimental system and protocol
The composting experiment used the in-vessel composting method and was conducted continuously for 15 days. The raw materials with 10 kg dry matter were put into a 55 L volume composting reactor (Fig. S1). The aeration mode was intermittent aeration and aeration of half an hour, stopping at half an hour, and the average aeration rate was 0.5 L/(kg DM • min). FW and sawdust were mixed well with the wet weight ratio of 7:1 to make the initial carbon to nitrogen ratio (C/N) be about 25. Moisture content (MC) was adjusted to about 65% by adding water. Six experimental groups with three replicates were designed as Table 1, and the amounts of additives in the table were based on the weight of raw materials. The pile was sampled and turned every three days. All the samples were homogenized using the methods of coning and quartering. Collected samples were stored in two parts. One part stored at 4 °C was used to determine physiochemical properties. The other part was stored at − 20 °C for microbial analysis.

Analytical methods
The temperature was recorded with a digital thermometer. The O 2 content was determined with a gas analyzer (Biogas 5000, Geotech). The MC was determined by the gravimetric method. The VS contents were analyzed according to the protocol of the American Public Health Association . The pH, EC, and GI value were determined with reference to the Chinese National Standard (NY 525-2021). TN content was tested by element analyzer (Germany). NH 4 + -N and nitrate nitrogen (NO 3 − -N) were measured by auto flowing analyzer (Germany). NH 3 content was adsorbed by boric acid (2%) and then quantified by titrating against 0.1 M sulfuric acid. N 2 O was collected daily from the reactor gas outlet by the syringe sampling method and then determined by a gas chromatograph (GC) (Beifen, China).
where X is the nitrogen loss rate (%); N 0 and N n are the TN content on days 0 and 15 (g/kg), respectively; and M 0 and M n are the weight of dry pile (kg) on days 0 and 15, respectively. The ratio of NH 3 emission to nitrogen loss (%) was calculated by Eq. (2).
where Y is the ratio of NH 3 emission to nitrogen loss (%); M 1 is the NH 3 accumulative emission (g); and M N is the weight of total nitrogen loss (g). (1) The ratio of N 2 O emission to nitrogen loss (%) was calculated by Eq. (3).
where Z is the ratio of N 2 O emission to nitrogen loss (%); M 2 is the N 2 O accumulative emission (g); and M N is the weight of total nitrogen loss (g).
The microbial community analysis was performed via high-throughput 16S rRNA gene pyrosequencing according to the method described by Luo et al. (2016). DNA was extracted using the Ezup genomic DNA extraction kit for soil (Fast DNA spin Kit). The V4 region of the archaeal and bacterial 16S rRNA gene was amplified using the universal primers 515F (5′-GTG CCA GCMGCC GCG GTAA-3′)/806R (5′-GGA CTA CVSGGG TAT CTAAT-3′).

Statistical analysis
Microsoft Excel 2016 was used for data analysis. The mean value and standard deviation of three replicates of each treatment were reported. SPSS 22.0 was used for all statistical analyses. Origin 2021 was used for Pearson's correlation analysis and graph production. Redundancy analysis (RDA) and forward selections were performed using Canoco for Windows (Version 5.0).

Physiochemical characteristics
Similar variations in composting temperature were observed for all groups (Fig. S2). The temperature of all experiment groups increased significantly to their summits (about 70 °C) on days 3-4 due to rapid decomposition of organic matter and then decreased gradually. A secondary increase in temperature was observed in all experiment groups on day 10, probably because the organics in anaerobic zone was decomposed for heat production after turning on day 9. In the later stage of composting, the temperature of all groups was maintained at 40 °C or below. After composting, the thermophilic phase (≥ 50 °C) lasted for all groups was all over 7 days and SB2 had the longest duration. Compared with the CK, the B1 and B2 warmed up faster and had higher peaks, reaching 71.7 °C and 68.8 °C on day 4, respectively, which might be due to the promoted microbial activity from abundant, exogenous microorganisms in the mature compost. However, the temperature in S was obviously lower than CK in the thermophilic phase. This was mainly related to the lower pH due to the addition of acidic additive SP, which might inhibit the microbial activity, thus delaying the heating rate of the pile in the early stage. During composting, microorganisms consumed O 2 to biodegrade organic substances to enhance the substrate temperature . A sharp decline and then gradual increase to the ambient level in O 2 content were observed for all experiment groups due to the intensive biodegradation and then depletion of easily organic matter, respectively (Xu et al. 2020) (Fig. S2). On days 3-4 of composting, the O 2 content of each group dropped to the lowest value. The O 2 content of B2 and SB2 reached the lowest values of 7.5% and 8.0% on day 3, respectively, and then remained at a lower level, suggesting higher strength of microbial aerobic metabolism due to the introduce of 2B.
The trend of pH changes remained basically the same for all groups, with a trend of rapidly rising tin the early stage then leveling off (Fig. S2). The initial pH of each group ranged from 4.69 to 5.21. Microorganisms multiplied and produced heat under sufficient nutrient conditions, which was favorable to ammonification. With the volatilization of NH 3 and the enhancement of nitrification, the pH of each group slightly fluctuated and finally stabilized. The pH of S raised slowly in the early stage, which was mainly due to the acidic SP. The EC values reflect the salt content in the pile. The changes of EC values were similar for each group, with an overall increasing trend (Fig. S2). In the early stage of composting, organic macromolecules decomposed and produced NH 4 + and small molecule fatty acids, leading to the gradually increased soluble ions (Pan et al. 2019). By the end of composting, the EC values of all groups ranged from 2.35 to 4.12 mS/cm, and the EC values of S, SB1, and SB2 were significantly higher than that of CK, probably because the addition of SP inhibited the conversion of NH 4 + to NH 3 and thus increased the EC values. The GI value of each group ranged from 43.2% to 127.8%. The final GI value of B2 showed significantly higher than others (p < 0.05) and 32.3% higher than that of CK (Fig. S2). In the composting process, GI increased and tended to become steady, as the toxic substances in the piles were gradually degraded (Pan et al. 2019). GI in the pile with adding SP raised slowly, and the lowest value at the end, suggesting that SP had an inhibitory effect on GI.
The variation of MC for different groups is shown in Fig. S2, and the initial MC of each group was 60-65%. The overall MC of each group showed a continuous decreasing trend in the composting process. The dewatering effect of each group was obvious on days 3-12, reaching 22.4%-38.9%. After 15 d of composting, the MC of each group ranged from 12.8% to 35.5%, and the MC removal rates ranged from 81.4% to 93.3%. Compared with CK, SP and mature compost amendments improved the MC removal rate, with the best dewatering effect in SB2. After composting, the VS degradation rates of CK, B1, B2, S, SB1, and SB2 were 38.9%, 42.7%, 44.1%, 35.6%, 37.9%, and 40.5%, respectively (Fig. S2). VS degradation rates of B2 and SB2 were higher than those of other groups, indicating that the addition of 2B significantly promoted the organic matter degradation.

Emission of nitrogenous gases
The NH 3 emission could be ascribed to the intensive mineralization of organic nitrogen to NH 4 + , which was further transformed into NH 3 under high temperature and alkaline conditions ). The NH 3 emission rate of each experimental group raised obviously since days 3-4, reaching the peak emission on days 5-6 ( Fig. 1a), and then decreased as the organic matter degradation rate became slower and the pile temperature started to decrease. The peaks of NH 3 emission rate in B1 and B2 were earlier than CK, and that of S was delayed, which was related to the increased temperature as mature compost was added and the decreased pH as the introduction of SP. The accumulative NH 3 emission decreased in the order CK > B1 > B2 > S > SB2, SB1 (Fig. 1b). NH 3 emission was more intense in the thermophilic stage of composting, accounting for 82.0%-88.1% of the total NH 3 emission. Compared with CK, all the piles with additives had a suppressive effect on ammonia emissions, and the accumulative NH 3 emissions of B1, B2, S, SB1, and SB2 were reduced by 7.05%, 18.62%, 32.73%, 34.69%, and 39.86%, respectively. It was reported that mature compost had fluffy structure and high adsorption capacity (Song et al. 2021), which might benefit the reduction of NH 3 emission. The decreased pH due to SP addition had more effect on reducing NH 3 volatilization compared to adding mature compost. The combined application of SP and mature compost could significantly reduce NH 3 emissions, with the best reduction of 39.86% for SB2 based on the physical and chemical reactions. Furthermore, the application of additives might promote the action of functional microorganisms, which had a key role in the conversion of NH 3 . (Jurado et al. 2014;Zhang et al. 2018).
The N 2 O emission mainly occurred in the initial and late stages of composting for all groups (Fig. 1c). There were 85.9%-93.0% of N 2 O emissions in the first week of composting. This was mainly due to the rapid increase in temperature at the beginning of composting, and the oxidation of organic nitrogen may promote the emission of N 2 O. It was reported that the nitrification gradually increases in the later stages of composting due to the drier substrate, good aeration conditions, and decreasing pile temperature (Guerra-Gorostegi et al. 2021), resulting in smaller N 2 O emissions. The accumulative N 2 O emission was decreased as S > CK > SB2, B2 > SB1 > B1 (Fig. 1d). All the additives except S could suppress N 2 O emission, which was reduced by 36.9%, 17.0%, 23.6%, and 15.7% for B1, B2, SB1, and SB2, respectively, compared to CK, and the addition of mature compost had the most significant inhibitory effect on N 2 O emission.

Nitrogen conversion and loss
An increase and then decrease of NH 4 + -N content were observed for all groups during composting, and the initial NH 4 + -N content of each group ranged from 0.20 to 0.42 g/ kg (Fig. 2a). In the early stage of composting, the NH 4 + -N content of each group started to rise and reached the peak on days 3-9 because nitrogenous organic matter was decomposed by microbial assimilation to produce a large amount of NH 4 + -N (Wang et al. 2017). Then, NH 4 + -N content decreased as the ammonification effect gradually weakened and a large amount of NH 4 + -N was converted into NH 3 and escaped (Moenne-Loccoz and Fee 2010). By the end of composting, the NH 4 + -N content of CK, B1, B2, S, SB1, and SB2 was 1.2708, 1.7554, 0.9579, 2.3784, 1.8368, and 0.698 g/kg, respectively and the peak of NH 4 + -N content was found in S, which was mainly due to the small NH 3 emission in the early stage of and the rate of NH 4 + -N production was greater than the conversion rate.
The initial NO 3 − -N content of each group ranged from 16.55 to 20.65 mg/kg (Fig. 2b). In the first 3 days, the NO 3 − -N content of each group showed a decreasing trend, which was mainly due to the significant denitrification of the pile in the early stage, and part of NO 3 − -N was converted to N 2 O and escaped. On days 3-9, NO 3 − -N content in each group showed a slight increase, but remained at a low level, which was mainly due to the inhibition of temperature-sensitive nitrifying bacterial activity during the thermophilic stage. Then, the pile temperature started to decrease and nitrification gradually increased, and the NO 3 − -N content showed an increasing trend on day 12-15. By the end of composting, NO 3 − -N content of CK, B1, B2, S, SB1, and SB2 was significantly different (p < 0.01), which increased by 81.6%, 98.3%, 87.8%, 58.4%, 127.5%, and 139.9%, respectively, compared with the initial NO 3 − -N content. The initial TN concentration ranged from 31.10 to 33.67 g/kg in each group (Fig. 2c). In the early stage, TN concentration of each group showed a decreasing trend due to the loss of nitrogen caused by the volatilization of large amount of NH 3 . As the pile entered the cooling stage, nitrification was enhanced and TN concentration slightly rebounded. The nitrogen loss rates of each treatment ranged from 29.9% to 49.3%, and SB2 had the lowest nitrogen loss rate (Table 2). Overall, CK had the largest nitrogen loss rate, which was mainly due to the high NH 3 emission and inhibited nitrification as the pile was maintained at a high temperature by the external heating method. All types of additives could reduce the nitrogen loss to different degrees. The ratio of N 2 O emission to nitrogen loss for each group ranged from  1.5% to 3.1%, and the ratio of NH 3 emission to nitrogen loss for each group ranged from 32.8% to 49.2%, meaning that NH 3 emission was the main factor causing nitrogen loss in the composting. Pearson's correlation analysis showed that during the composting, temperature integration (TI) index had a significant positive correlation with NH 3 emission ( r = 0.943, p < 0.01 ) and nitrogen loss ( r = 0.961, p < 0.01 ) (Fig. 3). This result suggested that the increase of the temperature facilitated the release of NH 3 and caused a greater nitrogen loss. Nitrogen loss was significantly and positively correlated with pH ( r = 0.810, p < 0.01 ), NH 3 emission ( r = 0.957, p < 0.01 ), N 2 O emission ( r = 0.816, p < 0.01 ), and negatively correlated to VS (r = −0.738, p < 0.01 ), and MC ( r = −0.805, p < 0.01 ), indicating that the temperature, the dewatering and capacity reduction effect of the pile significant affected, the transfoemation of nitrogen fractions during composting.

Dynamics of bacterial community and its potential functions
The dynamics of bacterial community from each experiment group were sequenced and analyzed, and 36 samples were screened to obtain 1,081,089 valid sequences, including 38 phyla, 82 orders, 146 species, 305 families, and 925 genera.
An increase and then decrease in all α-diversity indices (Chao 1 and Shannon index) occurred during the composting (Fig. S3). The highest Chao1 and Shannon index were found for B2 and SB2 on day 0 of composting, which was mainly related to the abundant microorganisms in 2B. The Shannon index of each additive group increased in the mesophilic stage of composting (days 0-3), which was due to the sufficient nutrients at the early stage of composting that were favorable to the growth and metabolism of microorganisms. When the pile entered the thermophilic stage, the Shannon index started to decrease, which was mainly due to the fact that heat-resistant bacteria started to dominate during the thermophilic stage, while other species died due to intolerance of high temperature. By the 15th day of composting, the Shannon index of each group increased again, because the thermophilic microorganisms in the pile began to recover. By the end of composting, the Chao1 index of S was the largest, which may be related to the postponed microbial decomposition process in the early stage of composting due to SP addition. The variation of beta diversity of bacterial community during composting is shown in Fig. S4. Principal component analysis (PCA) also showed the pattern of microbial community variation with time. The results showed that the variation due to different experiment groups and composting time variation explained 59.5% and 16.8% of the total variation of bacterial community, respectively, indicating that the effect of composting time on bacterial community structure was greater than that of treatments.
To clarify the effects of different additives on nitrogen loss in the composting, a hierarchical cluster analysis was conducted based on different nitrogen indicators (NH 4 + -N, NO 3 − -N, NH 3 emission, N 2 O emission, TN, and nitrogen loss) (Fig. S5), indicating that there were two major classes for six experiment groups, with SB2 as the first major class (A) and the remaining groups as the second major class (B). B could be divided into two subcategories, where the first subcategory was SB1 and the second subcategories were the remaining groups. The results of the sample hierarchical clustering analysis based on OTUs at different time points in the six experiment groups of test samples are shown in Fig. S5. In the six sampling periods of days 0, 3, 6, 9, 12, and 15 of composting, the bacterial communities of different Fig. 3 Pearson's correlation between physiochemical characteristics and gaseous emissions. TN, total nitrogen content; EC, electrical conductivity; VS, volatile solids; TI, effective accumulated temperature index; MC, moisture content; GI, germination index experiment groups formed different clustering branches. It could be seen that B2 and SB2 clustered into one group on day 0 of composting, and the rest of the groups clustered into one group, with S, B1, and SB1 clustered into one subgroup at a closer distance. A clear branch was formed between the CK and the rest of the additive groups at the middle 4 sampling times (day 3, 6, 9, and 12). By the 15th day of composting, the CK formed a separate cluster, B1 and B2 formed a branch, S and SB1 clustered into one category, and SB2 formed a separate cluster, which also corresponded to the cluster analysis of the nitrogen fraction content. The above results indicated that each additive changed the community structure of the flora in the pile, and all types of additives changed the living environment of microorganisms after adding to the pile, which in turn significantly affected the nitrogen loss and transformation of the pile, with SB2 showing the most obvious performance.
The succession of bacterial community during composting was examined by taxonomic analysis at the phylum level (Fig. 4). The dominant bacterial phyla mainly consisted of Firmicutes, Actinobacteria, and Proteobacteria. Previous studies suggested that these phyla were closely responsible for the biodegradation of organic substances and were detected ubiquitously in composting processes (Liu et al. 2018b;Wang et al. 2018a). In the mesophilic and thermophilic stages of composting, Firmicutes of each group was absolutely dominant, which is more related to the high heat tolerance of Firmicutes, and can grow rapidly under nutrient-rich conditions (Liu et al. 2018a;Wang et al. 2018a;Mao et al. 2019). As the pile temperature gradually decreased, the relative abundance of Firmicutes gradually decreased, while the relative abundance of Actinobacteria and Proteobacteria gradually increased and became dominant. Actinobacteria and Proteobacteria have been shown to be important phyla for the degradation Fig. 4 Variation of dominant bacterial community composition at phylum level (relative abundance > 1%) during composting. Phyla with relative abundance < 1% were defined as "others" of lignin, cellulose, and proteins, which can effectively degrade organic matter in the pile and have a major contribution to the degradation and decay of the pile (de Gannzs et al. 2013). Thus, it is clear that Firmicutes play a major influence on pile warming, while Actinobacteria and Proteobacteria play a major influence on decomposition during composting.
The succession of bacterial community during composting was shown at the genus level (Fig. 5). Among the top 20 genera in terms of relative abundance in all experiment groups, 13 genera belonged to Firmicutes and 5 genera belonged to Actinobacteria. The remaining 2 genera belonged to Proteobacteria and Cyanobacteria. At the beginning of composting, the dominant species in all groups were Lactobacillus in Firmicutes, with relative abundance ranging from 26.7% to 58.7%. Lactobacillus showed the highest relative abundance in CK on day 0. As the temperature increased, the relative abundance of Lactobacillus began to decrease. On days 3-6, the pile temperature increased to above 50 °C, and the relative abundance of Thermobifida of Actinobacteria and Ureibacillus of Firmicutes gradually increased as the dominant species. It is reported that Thermobifida and Ureibacillus, as two thermophilic bacteria, are the main microbial taxa that degrade organic matter at high temperature and have been detected in most of the high temperature periods of composting tests . The relative abundance of Ureibacillus and Thermobifida in S was low (< 2%) in the early stage and increased substantially in the later stage of composting, which corresponded to the low degradation rate in the early stage and the substantial increase of fermentation rate in the later stage (Fig. 6). In contrast, Ureibacillus and Thermobifida in B2 and SB2 contained higher abundance on day 0 compared to other experiment groups, which might be related to the thermophilic bacteria retained by the 2B after multiple fermentation screening. Combined with the efficient dewatering capacity of B2 and SB2 and the changes of degradation rate of S, it was speculated that Ureibacillus and Thermobifida were the key strains for the high dewatering and decomposition capacity of 2B.
The relative abundance of nitrogen-fixing bacteria such as Azotobacter, Azospirillum, and Paenibacillus in CK was low (0.1%-1.2%). In contrast, the relative abundance of nitrogen-fixing bacteria was higher in a composting process with different additives (0.3%-2.4%), with the highest in SB2. Bacillus and Pseudomonas are predominantly ammonifying and denitrifying bacteria, which are important for nitrogenous gas emissions as important drivers of ammonification and denitrification, and it was reported that changes in NH 3 and N 2 O were correlated with the number and activity of this genus (Otawa et al. 2006). The relative abundance of Bacillus and Pseudomonas in composting process in CK was higher than that of groups with additives (Fig. 6), leading the highest nitrogenous gas emissions and nitrogen loss in CK. Similarly, the relative abundance of these two genera in SB2 was lower than that of other groups, which might be the reason why the nitrogenous gas emissions and nitrogen loss in SB2 were the lowest. Taken together, additives reduced nitrogenous gas emissions and nitrogen loss by reducing the number and activity of these two genera. Towards the end of the composting, the temperature gradually decreased and the community diversity of the groups gradually recovered, with Saccharomonospora becoming the new dominant species in the experiment groups, while Lactobacillus almost completely disappeared.
In order to study the relationship between each nitrogen fractions and bacterial community, the association between nitrogen indicators and bacterial community was analyzed by redundancy analysis (RDA), as a way to clarify the main bacterial species affected by the use of additives on the transformation of nitrogen fractions. RDA showed that Firmicutes and Cyanobacteria were positively correlated with TN and negatively correlated with NH 4 + -N, nitrogenous gases (NH 3 , N 2 O), and nitrogen loss (Fig. 7a), while Actinobacteria and Proteobacteria were negatively correlated with TN and positively correlated with inorganic nitrogen  , nitrogenous gases (NH 3 , N 2 O), and nitrogen loss. These results indicated that the addition of additives had an important driving role for the conversion of nitrogen fractions in the composting process and could significantly affect the nitrogen loss and conversion process. As shown in Fig. 7b, the response of microorganisms to environmental factors in the composting process indicated that key microbes affecting different nitrogen fractions were significantly regulated by MC, which had the highest contribution to the change of bacterial community composition (p < 0.01).

Conclusions
The addition of chemical additives and mature compost effectively improved food waste composting by alleviating gaseous emissions and enhancing maturity. On 15 days of composting, the NH 3 emission was mitigated by up to 56.3% with the synergistic effects of chemical additives and mature compost addition (SB2), and the B1 was the most effective for N 2 O emission reduction (36.9%). SB2 achieved the lowest nitrogen loss and B2 achieved the highest GI value. The temperature, pH, and the release of nitrogenous gases significantly affect the nitrogen loss during composting. The relative abundance of nitrogen-fixing bacteria was higher, and the relative abundance of predominantly ammonifying and denitrifying bacteria was lower than CK, with the addition of different additives. Taken together, the combined addition of mature compost and sulfur powder can not only improve the maturity of FW composting but also reduce nitrogen loss.